Daytime Laser Radar Measurements of the Atmospheric Sodium Layer

Abstract

IT is generally accepted that the column density (abundance) of atomic sodium in the layer at about 90 km altitude is enhanced during the day by a factor of 2 to 6 relative to that at twilight. The evidence for the enhancement is chiefly provided by ground-based observations of the sodium D line dayglow^1–5, but there has been some discussion^4–6 of the possibility that the ring effect (the partial filling of the Fraunhofer lines in scattered sunlight) may have affected the measurements. Observations of the dayglow during 6 rocket flights^7–9 and spectrometer observations of the absorption of direct sunlight by sodium^10–12 have produced various conflicting results concerning the daytime enhancement. Recently, however, Burnett et al.¹³ have published the results of absorption measurements at latitude 26° N giving daytime abundances which, when compared with twilight measurements made at a similar latitude some years previously, are clearly much lower than would be expected if the enhancement were present. In an independent effort to resolve the question of the daytime enhancement we have developed further the technique of resonance scattering from a laser pulse tuned to the D line¹⁴ to measure the sodium concentration during the day. Our previous measurements^15,16 have shown that the night-time abundance is within 20% of that at twilight and we now find no evidence of a daytime enhancement.